Ti-Ni Phase Diagram with Applied Stress
- PDF / 296,906 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 30 Downloads / 191 Views
Ti-Ni PHASE DIAGRAM WITH APPLIED STRESS A.Peter Jardine Dept.of Materials Science and Engineering, S.U.N.Y. at Stony Brook, Stony Brook, NY 11794-2275. Abstract The role of stress on the phase evolution of thin-film TiNi has not been investigated and may play an important role in the phase evolution of thin film TiNi. In this paper, a preliminary set of phase diagrams for Ni-Ti at different pressures are presented relating the stability of the stoichiometric TiNi phase to the other well-documented intermetallics TiNi3 and NiTi2. It is found that for sufficient pressure of the order of a GPa, the region where NiTi (/3) phase is single phase shifts towards the Ti-rich side of the diagram. The implications on the annealing of TiNi thin film is discussed. Introduction The development of SME TiNi is dependent on both processing conditions and on the chemical interactions with the substrate. In this paper, we focus on the problems involved with the application of stress. This investigation was initiated by the high degree of variability of the final microstructure of thin film TiNi deposited on Sapphire. Using a UHV deposition system under identical controlled conditions, similar amounts of amorphous, stoichiometric TiNi were deposited on room temperature Sapphire. However annealing at 5000 and at nominally 6000C results in far different crystal structures. As evident from the TiNi phase diagram [1] shown in Figure 1, annealing at 5000C implies that for a stoichiometric composition, B2 is the stable phase. However, Johnson et al have observed in thin film TiNi that although B2 is observed initially, annealing for one hour will allow formation of intermetallic phases [2]. It is evident that the conventional phase diagram may not be adequate to explain the variability in the final microstructure. Typcially, thin film TiNi are deposited either onto nominally room temperature substrates followed by subsequent annealing or deposited and crystallized simultaneously onto heated substrates. As-deposited thin-film TiNi onto room temperature substrates is amorphous. On heating to crystallization temperatures of 500' to 6000 C, the thin film can experience significant strains, due to lattice mismatching, thermal expansion mismatches, and volume contractions on annealing. Scanning Electron Microscopy of thin film TiNi annealed at 6000C on sapphire, revealed pits and bubbles in the thin-film, consistent with large strains due to annealing. The stresses which can tear the material, therefore resulting in stresses of the order of several GPa. Thus as the material is annealed, it may experience stresses of several GPa. The conventional phase diagram for Ni-Ti is normally presented at a fixed hydrostatic pressure of 1 atm. Considerable doubts remain concerning the nature of the Ni-Ti phase diagram at near the stoichiometric composition. The phase diagram given in [1] indicates that the TiNi phase decomposes by a eutectic decomposition into TiNi 3 and Ni 2Ti at 900K. Wasilewski et al [3] have observed intermetallic precipitates, not indic
Data Loading...
